1) Field of the Invention
The present invention relates to the forming and bonding of structural members and, more particularly, relates to the use of different grain titanium for superplastic forming and/or diffusion bonding.
2) Description of Related Art
Superplastic forming (SPF) generally refers to a process in which a material is superplastically deformed beyond its normal limits of plastic deformation. Superplastic forming can be performed with certain materials that exhibit superplastic properties within limited ranges of temperature and strain rate. For example, workpieces formed of titanium alloys are typically superplastically formed in a temperature range between about 1450° F. and 1850° F. at a strain rate up to about 3×10−4 per second.
Diffusion bonding (DB) generally refers to a process of joining members using heat and pressure to form a solid-state coalescence between the materials of the joined members. Joining by diffusion bonding occurs at a temperature below the melting point of the materials that are being joined, and the coalescence therebetween is produced with loads below those that would cause macroscopic deformation of the article.
According to one conventional process, superplastic forming is performed by providing one or more superplastically formable metal sheets in a die cavity defined between cooperable dies, heating the sheets to an elevated temperature at which the sheets exhibit superplasticity, and then using a gas to apply differential pressures to the opposite sides of the sheets in order to form the sheets. The pressure is selected to strain the material at a strain rate that is within its superplasticity range at the elevated temperature, stretch the sheet, and cause it to assume the shape of the die surface. In this way, the sheet can be formed to a complex shape defined by the dies.
Further, in some cases, superplastic forming and diffusion bonding are performed in a combined forming/bonding operation. For example, in one typical combined SPF/DB process, three metal sheets are stacked to form a pack. A stop-off material is selectively provided between the sheets to prevent portions of the adjacent surfaces of the sheets from being bonded. The pack is heated and compressed in a die cavity with sufficient gas pressure so that the adjacent portions of the sheets that are not treated with the stop-off material are joined by diffusion bonding. Thereafter, a pressurized gas is injected between the sheets to inflate the pack, and thereby superplastically form the pack to a configuration defined by the surface of the die cavity. This process is described further in U.S. Pat. No. 3,927,817 to Hamilton, et al. Such a combined SPF/DB process can be used, e.g., to produce complex honeycomb sandwich structures that are formed and diffusion bonded to define hollow internal cells. Generally, the simplicity of the superplastic forming and/or diffusion bonding processes can result in lighter and less expensive structures with fewer fasteners and higher potential geometric complexity. Applications of SPF and/or DB include the manufacturing of parts for aircraft, other aerospace structures, non-aerospace vehicles and structures, and the like.
The individual sheets of a pack for forming according to the foregoing conventional process are typically provided as a flat sheets in a stacked relationship. FIG. 1 illustrates a portion of a three-sheet pack after being diffusion bonded and superplastically formed according to the conventional process. As shown, the space S between the outer sheets (or “face sheets”) F1, F2 has been expanded by the gas and the middle sheet (or “inner sheet” or “core sheet”) C has been superplastically formed to a corrugated or otherwise non-linear shape so that the middle sheet C extends in alternating directions between the outer sheets F1, F2 and defines the cells of the pack. As the outer sheets are expanded outward, the middle sheet tends to exert a reactive force on the outer sheets, thereby causing the outer sheets to be deformed. The effect of this reactive force is shown in FIG. 1 as deformation of the outer sheet where the middle sheet is connected thereto. In particular, instead of the outer sheet defining a flat surface, the outer sheet has been deformed to form a depression M, typically referred to as “markoff,” on its surface opposite the connection to the middle sheet.
Such markoff of the outer sheets of a pack during superplastic forming can be reduced by providing a middle sheet that is significantly thinner than the outer sheets, thereby increasing the relative stiffness of the outer sheets and reducing the inward force on the outer sheets during forming. The ratio of the thickness of the middle sheet to the thickness of each outer sheet is typically no more than about 25%. Therefore, if the design requirements for a particular application require a thicker middle sheet, superplastic forming is not typically used. The production of two-sheet assemblies and assemblies having other numbers of sheets can similarly be limited by a desire to avoid markoff.
While the conventional methods for SPF/DB processing have proven effective for manufacturing a variety of structural assemblies, including assemblies formed of titanium, there exists a continued need for improved SPF/DB methods and assemblies. In particular, the method should allow the production of assemblies with a greater range of desired dimensions, including assemblies with sheets of particular dimensions.